This invention relates to a method for production of cement clinker from raw meal, which is preheated in at least one heat-exchanger train through which the off-gas of a rotary kiln flows or is precalcined in a precalcination stage and, in the sintering zone of the rotary kiln, is burned to cement clinker, which is cooled in a downstream cooler, an unpreheated additive usable in the cement clinker production process, such as for example slag, being added to the process meal. The invention further relates to an installation for the performance of the method.
In the production of cement clinker it is known to use, as a raw material component, slag arising in steelmaking, by introducing the slag directly into the stock inlet of the rotary kiln. The slag melts in the sintering zone of the rotary kiln and is there incorporated into the cement clinker being formed, which is then ground into portland cement.
On the other hand, it is known to grind latently hydraulic substances and other additives, for example granulated blast-furnace slag, jointly together with portland cement clinker into slag portland cement, usually in a ball mill, separate grinding and subsequent homogeneous mixing also being possible. The slag is often more difficult to grind than the cement clinker, so that it is known to grind the slag separately per se, for example in the nip of an energy-saving autogenous high-pressure roll press, and in this way to reduce the energy consumed in grinding and to achieve the optimal grinding fineness for each of the two grinding products.
Modern closed-circuit grinding installations with autogenous high-pressure roll presses and three-fraction classifiers, such as shown in U.S. Pat. No. 5,392,998 issued Feb. 28, 1995 to Albert Sussegger et al for a Sifter for Sifting Granular Material and Grinding System with Insertion of such a Sifter, have the principal aim of producing two cement products unequal in fineness in cement grinding at comparatively low cost in terms of investment and operating costs. They also make it possible, however, to grind grinding stock such as slag into the fines fraction or finished stock fraction carried off with the classifying air, into an oversize fraction recirculated to the roll press, and into an intermediate fraction as a second finished product of intermediate fineness. While the fines fraction, as finally fine slag meal, could be conveyed to a slag-meal bunker for subsequent mixing with the separately ground cement clinker in accordance with a specified formula in order to obtain granulated blast-furnace slag cement or slag cement, there has heretofore been no suitable use in cement production for an intermediate slag fraction that is coarser than cement. On the other hand, the production of the final fine slag meal in closed circuit slag grinding is greatly facilitated by the withdrawal of an intermediate fraction from the grinding circuit, because the grinding of slag or of granulated blast-furnace slag is energetically costly as a consequence of the difficult grindability.
In the production of cement clinker or cement with the addition of a cement additive such as slag, which is ground in a closed-circuit grinding installation to obtain slag meal of equal fineness to cement, it is an object of the invention to utilize excess slag capacity in a beneficial way in the cement clinker production process.
According to the invention, this object is achieved by the herein described method and equipment.
The invention proceeds from the notion of grinding additives usable in cement production, such as for example the slag arising in iron making or steel making, pozzolans, limestone or trass, separately from the cement clinker because of their difficult grindability. According to the invention it is now proposed to grind such difficult to grind additives in a closed-circuit grinding installation with mill and three-fraction classifying device, a so-called intermediate fraction with a fineness of, for example, 100% less than 200 xcexcm being withdrawable as a second product from the classifying device as a third fraction, in addition to the fines fraction having, for example, 100% less than 32 xcexcm and in addition to the oversize fraction (grit) recirculated to the mill. If in such a three-fraction classifying device this intermediate fraction is withdrawn from the grinding circuit, the throughput capacity of the closed-circuit grinding installation, for example slag grinding installation, can be enhanced quite substantially.
While the fines fraction withdrawn from the three-fraction classifying device is conveyed to a bunker as a product equal in fineness to cement, with the possibility of subsequent mixing with ground cement clinker in order to obtain mixed cement, the intermediate fraction withdrawn from this classifying device is added as a valuable raw material component to the process meal of the cement clinker production line, specifically by introduction into the inlet chamber of the rotary kiln and/or metering into the feedstock of the rotary kiln and/or to the calcinator and/or to the heat-exchanger train. The added quantity of the intermediate fraction from slag grinding replaces a portion of the cement raw material without having to be ground to the fineness of cement raw meal. The chemical composition of the remaining raw material components is adapted to the added quantity of the intermediate slag fraction.
The advantages associated with the invention are briefly summarized as follows:
Completely separate grinding of the unequally grindable substances, for example slag, pozzolans, trass, etc., on the one hand and cement clinker on the other hand. The additives can also be ground more finely than the cement clinker in the interest of optimal strength development of concrete prepared with the cement.
Enhancement of the throughput capacity of the closed-circuit slag grinding installation through removal of an intermediate fraction from the grinding circuit.
High moisture content, for example of granulated slags, does not upset the grinding process, because moist slags can be dried in the three-fraction classifying device.
Utilization of the intermediate slag fraction as a valuable raw material component in cement clinker production. Because of the similar chemical composition, silicate components of the raw-meal mixture (e.g., clays, sands, shales or bauxites) can be wholly or partly replaced by such an additive.
Because the intermediate slag fraction contains only a low level of moisture, hardly any water of crystallization and hardly any carbonates, practically no drying/dehydration and no calcination is necessary; thus addition of the intermediate fraction to the process meal in the region of the stock inlet of the rotary kiln, with bypassing of the heat-exchanger train if appropriate and of the calcinator if appropriate. Thus lower specific heat or fuel consumption in cement clinker production and lower emission of pollutants, such as for example also CO2, in the off-gas.
Cooling of the rotary kiln inlet chamber, which is subject to severe thermal stress, by the introduced cold, unpreheated intermediate slag fraction, and thus enhancement of operational reliability.
The cold particles of the intermediate slag fraction introduced into the rotary kiln inlet chamber can act as condensation nuclei on which pollutants in gas or vapor form contained in the rotary kiln off-gas, such as in particular, for example, alkali chlorides and alkali sulfates, can condense (freeze out or sublime), and which can then be extracted from the kiln circuit if appropriate by gas substream extraction.
The closed-circuit grinding installation for the grinding of the additive, such as for example slag, advantageously has the energy-saving high-pressure roll press with autogenous size reduction of the slag material. The scabs of stock pressed in the roll press are then deagglomerated and classified into three fractions. The classifying device advantageously includes a three-fraction classifier of compact design with a static cascade classifier as preclassifying chamber, downstream of which is at least one rod cage of a dynamic classifier section, the rod cage being rotatably arranged in the classifier housing and provided with turbo elements on the periphery of the rotor, on the periphery of which rod cage the discharge of the intermediate slag fraction is arranged.
The static cascade classifier section has a V-shaped shaft-shaped housing with two classifying-zone boundary walls enclosed by the classifier housing and forming between them a classifying zone, and through which boundary walls classifying air or hot gas flows approximately in crosscurrent as drying gas, which boundary walls exhibit cascade-shaped or louver-shaped guide vanes obliquely inclined downwardly in the direction toward the discharge opening for the oversize classified fraction, these two guide-vane walls and the classifying zone lying therebetween being arranged obliquely at an angle departing from the vertical.
While the discharge of the intermediate fraction of the classifying device is connected, either directly or, for example, via truck hauling, for example to the stock inlet region of the rotary kiln, the discharge of the fines fraction of the ground slag, equal in fineness to cement, can be connected to the bunker for subsequent mixing with ground cement clinker in order to obtain mixed cement.